Doors stick closed

VARIEZE

1998 VOLKSWAGEN JETTA

290,000 MILES

My 1998 jetta TDI front doors sometimes stick closed, the inside/outside door handles operate, but the door will not open. The dashboard mounted locking switch makes the door locks go up and down, but the door (or doors) still won, t open. Usually in cold weather. Sometimes they would work a hour later. What is wrong?